Hepatitis A virus (HAV) is an important cause of human hepatitis. In the United States it has been estimated that over 100,000 clinical cases occur annually. HAV continues to be endemic in underdeveloped areas of the world where infections usually occur in children, and nearly all of the young adult population have antibody to HAV (anti-HAV). Clinical hepatitis A and prevalence of anti-HAV are decreasing in industrialized nations, resulting in increasing numbers of adults susceptible to infection.
HAV is spread predominately by the fecal-oral route. Spread of hepatitis A is usually associated with overcrowding, poor hygiene, or breakdown in normal sanitary conditions. Contaminated food or water are frequent vehicles of spread. Groups at high risk include institutionalized persons, contacts of very young children in day-care centers, male homosexuals, consumers of raw shellfish and travelers to areas of the world where the disease is endemic.
The host range of HAV is limited to man, apes (especially the chimpanzee), and several species of New World monkeys. The incubation period for natural infections with HAV in man ranges from 15 to 45 days and averages 25 days. The first serological marker to appear is HAV in the stool, which often occurs 7-10 days before the onset of symptoms (dark urine or jaundice). Viral replication appears to be limited to the liver and excretion into the stool, where the highest levels of infectious virus are found, probably occurs via the biliary system. The virus is often rapidly cleared after the onset of symptoms and becomes undetectable in the stool. However, in some individuals, HAV can be found in the stool for longer periods.
Radioimmunoassay is a sensitive technique for detecting HAV in stool samples during the period of excretion, but it is not used in most clinical laboratories because assays for anti-HAV in serum (described below) are more easily performed and accurate. Therefore, patients with hepatitis A are usually considered as potentially infectious for up to two weeks after the onset of jaundice. Hepatitis A usually resolves with weeks, but occasionally illness may persist for several months. Mortality from hepatitis A or associated chronic liver disease are very unusual occurrences. Anti-HAV is almost always detectable in serum when symptoms begin. Because of this, diagnosis of hepatitis A is established using commercially available assays that are based on detection of anti-HAV IgM. An example of such an assay is that produced and marketed by Abbott Laboratories, North Chicago, Ill. under the name HAVAB-M.TM. kit. The development of anti-HAV IgG appears to be associated with lifelong immunity to HAV, for which only one serotype has been described. Temporary protection against hepatitis A for susceptible individuals can be achieved by injection of immune serum globulin, but at present there is no vaccine available.
The 27 nm virion of HAV was first visualized in 1973. See, Feinstone, S. M., Kapikian, A. Z. & Purcell, R. H. (1973) Science 182, 1026-1028. It was first isolated in tissue culture in 1979. See, Provost, P. J. & Hilleman, M. R. (1979) Proc. Soc. Exp. Biol. Med. 160, 213-221. Recently, HAV has been classified as a picornavirus. See, Coulepis, A. G., Locarnini, S. A., Westaway, E. G., Tannock, G. A. & Gust, I. D. (1982) Intervirology 18, 107-127.
HAV has a sedimentation coefficient of approximately 160 S and a primary buoyant density of 1.34 g/ml in CsCl. Virion capsid polypeptides of M.sub.r =32,000, 26,000, 22,000 and 10,000 have been described. The single-stranded infectious RNA has a molecular weight of about 2.5.times.10.sup.6 ; various genome lengths (between 6700-8100 nucleotides) have been reported. It contains poly(A), presumably at the 3' terminus. By analogy with other picornaviruses, the RNA should contain an open reading frame of about 6500 nucleotides which directs synthesis of a polyprotein that is post-translationally cleaved into virion proteins. These include the four capsid proteins, a peptide linked to the 5' end of the genome (VPg), an RNA-dependent RNA polymerase, and a protease. Putnak, J. R. & Phillips, B. A. (1981) Microbiol. Rev. 45, 287-315; Kitamura, N., Semler, B. L., Rothberg, P. G., Larsen, G. R., Adler, C. J., Dorner, A. J., Emini, E. A., Hanecak, R., Lee, J. J., van der Werf, S., Anderson, C. W. & Wimmer, E. (1981) Nature (London) 291, 547-553; Racaniello, V. R. & Baltimore, D. (1981) Proc. Natl. Acad. Sci. USA 78, 4887-4891; and, Nomoto, A., Omata, T., Toyoda, H., Kuge, S., Horie, H., Kataoka, Y., Genba, Y., Nakano, Y. & Imura, N. (1982) Proc. Natl. Acad. Sci. USA 79, 5793-5797.
The genomes of wild-type HAV strain HM-175 and its cell culture-adapted (CC) variant have been cloned as cDNAs in front of the Sp6 promoter of a plasmid expression vector (Cohen et al J. Virol. 3:5364 (1989); Cohen et al J. Virol. 61:3035 (1987)). In vitro transcription of the cDNA clone of the CC variant produced an RNA which was infectious when transfected into cultured primary AGMK cells, and the resultant virus displayed the growth and attenuation phenotypes of the parent (Cohen et al J. Virol. 63:5364 (1989); Cohen et al, J. Virol. 61:3035 (1987)). Although RNA transfected from the wild-type cDNA clone was not infectious in these assays, certain chimeric genomes containing wild-type sequence in combination with the P2/P3 sequence of the CC variant were infectious (Cohen et al, J. Virol. 63:5364 (1989)). The construction and analysis of additional chimeric genomes from these two cDNA clones has been reported and it has been demonstrated that mutations in both the P2 and 5' noncoding region of the genome are capable of increasing the efficiency of virus growth in vitro (Emerson et al, J. Virol. 65:4882 (1991). The effects of mutations in the 5' noncoding region are apparently host cell dependent, whereas those of the P2 region appear to be host cell independent.